The present invention relates generally to brazing methods and alloys and more particularly to brazing methods and alloys for use on bismuth steel.
Brazing is a method used to join together two steel parts. The brazing medium is usually in the form of wire composed principally or entirely of copper, or of paste having a metallic part composed principally or entirely of copper, together with an organic binder.
The brazing process is conducted at an elevated temperature above the melting point of the metallic component of the brazing medium (e.g., copper, which has a melting point of 1984.degree. F. (1084.degree. C.)) but below the melting point of the steel parts. During the brazing process, molten brazing metal becomes interposed as a film or layer at the interface between the two steel parts to be adhered together, and the entire assemblage is then allowed to cool to room temperature during which the brazing metal constituting the interfacial film solidifies, causing the two steel parts to adhere together at a brazing joint defined by the solidified brazing metal. In a typical brazing process, the assemblage is heated at a temperature above the melting point of the brazing metal for about fifteen minutes, and then allowed to cool for about one hour.
Oftentimes, one or both of the steel parts which are to be adhered together by brazing is a machined part, and in such instances it is frequently desirable that the machined part be composed of a steel having good machinability qualities. Such steels are known as free-machining steels, and a typical example of a free-machining steel which has been frequently used in the past is a leaded steel. However, there are health hazards associated with the production of a leaded steel, and, more recently, bismuth steels have been utilized in lieu of leaded steels.
However, when a machined part composed of bismuth steel is to be brazed to another steel part, a problem arises which was not present when the machined part was composed of leaded steel. More particularly, it has been found that when a bismuth steel part is brazed to another steel part, the brazing joint is brittle and may fail under service conditions substantially less severe than those at which the individual steel parts fail.
The underlying cause of the embrittlement was determined in the course of the development of the present invention. More particularly, at the elevated temperature at which the brazing process was conducted, the bismuth in the bismuth steel melts and diffuses into the brazing joint where it is dissolved into the copper from the brazing metal. As the copper cools, during the cooling part of the brazing cycle, it solidifies before the bismuth does, and the bismuth is rejected from solution by the copper. Because bismuth has a much lower melting point than copper, the bismuth is still molten as the copper solidifies, and the molten bismuth tends to segregate at the grain boundaries of the copper brazing joint. The segregation of bismuth at these grain boundaries is what creates the embrittlement of the brazing joint.